This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons folder. You should end up with a directory structure similar to below:
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
toffeemallows, kefir4uk, 3543450398, Zatchfine, optimusmikee, Tomekiac78, Nephilim, nantawtgolf2499, lizzymtz2, desinteresado124, kitsbaby, [KR] Doge, 1228256275, Марио и Луиджи 44422, HandsomePalpi194, Burger, cobra, ᠌᠌, Porter Justice, laureanotymczyszyn, Rtkz_wasa1974, Wolfish EL CUCARACHO, FiLzO, NuLL978, ToastMuffin, iRAGE, Deika, CONRECON, X dude Awesome, MarioDox, Spastik